Heat treatment of steels - II

By:Nishant S. KhatodAssistant Professor

STC, Latur

Introduction Components like ball and tapered bearings, gears, rock drill bits, camshafts,

crankpins, axles require outer surface to be hard and wear resistant and inner core more ductile and tougher

Such properties ensure long service life and sufficient toughness to withstand shock loads

Ways to achieve such combination of properties: Thermochemical treatment – Change in surface composition by diffusion of

C and N2 – Carburising and Nitriding Phase transformation of outer surface by rapid heating and cooling –

Flame, Induction, electron, and laser beam hardening

Carburising Also known as cementation OR case carburising OR case hardening Carburising – Method of increasing carbon Applicable: Low carbon steels with % C = 0.1 to 0.25 Process: Heating to 900 to 930ᵒC in presence of solid, liquid or gas rich in carbon Holding for definite period till desired case depth is achieved Cooling Carbon diffused into steel when heated in austenitic region Surface layer enriched with 0.7 to 0.9% carbon Fully austenitic steel is essential because solubility of carbon is more in

austenite than in ferrite Depending upon the medium, it can be classified as; Solid OR Pack OR Box carburising Liquid carburising or salt bath carburising Gas carburising Vacuum carburising

Solid carburising Components to be carburised packed with a compound rich in carbon in steel or

CI boxes and sealed with clay If not sealed properly medium comes in contact with air and burns without

carburising Medium: 50 to 55% hardwood charcoal, 30 to 32% coke and remaining energiser

or accelerator like BaCO3

Process: Heating boxes in a furnace upto 930ᵒC Holding for definite period till required case depth is achieved Cooling High temperature helps in aborption of carbon on surface Reactions:

O2 (from box) + C (medium) CO2

BaCO3 BaO + CO2

CO2 + C (from medium) 2CO2CO + Fe Fe (C) + CO2

Liberated CO2 + C (from medium) 2CO

Solid carburising This is indirect carburising Direct carburising: Carburising @ steel in direct contact with medium, Not

desirable because of local variations; Non uniform hardness. Maximum carbon @ the surface and case depth depend upon temperature

of carburising and holding time Higher the temperature, higher is the case depth but grain coarsening occurs Higher the holding time, higher is the case depth without change in maximum

concentration at the surface Carburising time of 6 to 8 hrs for case depth of 1 to 2mm @ 900ᵒ C Used when extreme uniformity in carbon content is not desired NOTE: Case depth: The perpendicular distance from the surface of the steel to the

point at which change in hardness, chemical composition or microstructure of the case and core cannot be distinguished

Liquid carburising Also known as salt bath carburising Carburising done by immersing the steel components in a carbonaceous fused

salt bath medium containing sodium or potassium cyanide, sodium and potassium chloride and barium chloride which acts as a activator

Bath heated in the range of 815 - 900ᵒC Reactions:

BaCl2 + 2NaCN Ba (CN)2 + 2NaClBa (CN)2 + Fe Fe (C) + BaCN2 (Barium cynamide)

Some beneficial nitrogen may also diffuse through oxidation of sodium cyanide

2NaCN + O2 2NaCNO3NaCNO NaCN + Na2CO3 + C + 2N

Nitriding helps in increasing hardness and wear resistance Carbursing time of 0.5 to1 hour for case depth of 0.1 to 0.5mm (Relatively

thinner than pack carburising) @ 900ᵒ C

Liquid carburising Advantages Uniform and rapid heat transfer Low distortion Negligible surface oxidation High uniformity in case depth and carbon content Disadvantages Highly poisonous sodium cyanide; hence care should be taken while storage,

use and disposal Salt sticks to the components and must be removed while washing

Gas carburising Components heated in the range of 870 to 950ᵒ C in the presence of carbonaceous

gases like methane, ethane, propane or butane diluted with a carrier gas containing 40% N2, 40% H2, 20% CO, 0.3% CO2, 0.5% CH4, 0.8% water vapor and traces of oxygen

Reactions: C3H8 2CH4 + C [Cracking]

CH4 + Fe Fe (C) + 2H2

CH4 + CO2 2CO + 2H2

2CO + Fe Fe (C) + CO2

Carburising mainly occurs due to CO to CO2 conversion H2 reacts with CO2 and increases CO concentration

H2 + CO2 CO + H2O Traces of oxygen are also present due to the following reactions;

2CO2 2CO + O2

CO2 + Fe Fe (C) + O2

To avoid dead spots and formation of soot: Control on gas composition and proper circulation of gas is essential for constant and uniform rate of carbon diffusion

Gas carburising Carburising time of 1 to 2 hrs for case depth from 0.2 to 0.5mm @ 900ᵒ C Suited for large volume production Accurate control on case depth and surface carbon content Less labor cost but skilled labor required for accurate control

Vacuum carburising Medium: Vacuum or Reduced pressure Two stage process:1. Carbon made available for absorption Component introduced in a furnace Furnace evacuated till required degree of vacuum Heated in the range of 925 to 1050ᵒ C Gaseous hydrocarbon like methane or ethane introduced in the furnace.

Amount of hydrocarbon depends upon size of component, surface area to be carburised, case depth and concentration of carbon to be introduced

Gaseous hydrocarbon cracks when comes in contact with surface which results in extremely fine carbon deposition on surface

Process continues till sufficient amount of carbon is absorbed Inflow of gas is stopped and excess gas removed by vacuum pumps2. Controlled diffusion cycle commences and continues till required carbon

concentration is formed and required case depth is achieved

Vacuum carburising Oil quenching is used Advantages: Components are free from oxides, microcracks and decarburization Energy saving process Disadvantages: Limited to batch type production Limitation on size of workpiece due to limited size of vacuum furnace Reasons for energy saving: Heating is carried out by radiation , improved efficiency due to vacuum Heat zones occupy less volume Not necessary to keep the furnace ON throughout the process. Absence of atmosphere Only 1% gas required compared to conventional process

Post carburising treatment Need for post carburising: Overheating may occur due to high carburising temperatures which results in

grain coarsening throughout the c/s Objectives of post carburising: Improve microstructure and refine grain size of core and case Achieve high hardness at the surface Break carbide network which may be formed due high carbon content (1% C) Following heat treatments can be used ; Direct quench Double quench Other quenching cycles

Cyaniding Applicable to steels with 0.3 to 0.4% C Surface hardened by addition by addition of carbon and nitrogen Process: Medium: Parts immersed in liquid bath containing NaCN varying between 25% and

90% Bath heated in the range of 800 to 960ᵒ C Measured amount of air passed through the molten bath Reactions:

2NaCN + O2 2NaCNO2NaCNO + O2 Na2CO3 + CO + 2N

2CO CO2 + C C and N2 so formed diffuse into steel and give thin wear resistant layer of carbonitride ϵ

phase Quenched in oil or water Low temperature tempering Cyaniding time of 1.5 to 6hrs for case depth of 0.13 to 0.35mm @ 850ᵒ C Higher the temperature, higher the C diffusion (0.8 to 1.2%) on surface as compared to

N (0.2 to 0.3%) Case hardness: 850VHN

Cyaniding Advantages: Less time consuming Less distortion due to use of salt bath Disadvantage: Not suitable for components subjected to shock, fatigue and impact because

nitrogen has adverse effect on these properties Difference between cyaniding and liquid carburising: Absence of alkaline earth salts in cyaniding High % of NaCN in case of cyaniding High N and lower C in case of cyaniding Thin cases in case of cyaniding

Carbonitriding Also known as dry cyaniding, gas cyaniding, and ni-carbing Applicable to steels with 0.3 to 0.4% C Surface hardened by addition by addition of carbon and nitrogen Used to improve wear resistance of mild steel and low alloy steel Process: Medium: Gas mixture consisting of 15% NH3, 5% CH4 and 80% neutral carrier gas Heated in the range of 800 to 870ᵒ C C and N2 diffuse into steel Quenching in oil to avoid cracking Tempering @ 150 to 180ᵒ C Case depth : 0.05 to 0.75mm Case hardness: 850VHN Nitrogen is more effective in increasing hardenability as compared to carbon Nitrogen content depend upon ammonia and temperature Advantages: Surface hardenability, wear resistance and corossion resistance better than carburising Disadvantage: Longer times than carburising

Nitriding Applicable to alloy steels containing nitride forming elements like Al, Cr, Mo,

V and W Process carried out @ 550ᵒ C; hence no phase transformation Proper heat treatment necessary before nitriding All machining and grinding operations to be completed before nitriding Area not to be nitrided to be covered by depositing tin by electrolysis Two types: Liquid nitriding: Same reactions as that of liquid carburising except only N

diffusion because of low temperatures Gas nitriding: Anhydrous ammonia gas is passed which dissociates into nascent

nitrogen and hydrogen2NH3 2 N + 3H2

Nitriding of alloy steels: Fe4N (White layer) + alloy nitrides (dark). Hence YES Nitriding of plain carbon steels: Only white layer. Hence NO Treatment time depends upon case depth and size; usually 21hrs to 100hrs Nitriding time of 100hrs for case depth of 0.5mm @ 550ᵒ C Case hardness: 900-1100VHN Achieved properties: Good wear resistance, hot hardness, corossion resistance

Nitriding Applications: Precision gears, boring bars, forming rolls for paper and rubber,

forming dies, camshafts, crankshafts, cylinder liners Advantages: No post heat treatment; hence minimum distortions High fatigue life Better corrosion resistance than carburised and hardened components Excellent bearing properties (Non metallic nature of nitrides, less coefficient of

friction) High hardeness than carburised and hardened components High hot hardness Disadvantages: Applicable only to alloy steels containing nitriding elements Thin case depth White layer No heat treatment can be done after nitriding

Nitriding

Plasma nitriding Also known as ion nitriding process What is plasma?? introduction to plasma.mp4 Component acts as cathode Process: Apply high DC voltage 500-1000V Electrically heated in the range of 370 to 650ᵒ C Gas mixture of N2 and H2 supplied at 1-10 torr Current flows and forms ionised gas Nitrogen ions bombard on the surface of component Part of energy heats the component and allows diffusion

of nitrogen and other part cleans the surface by displacing secondary electrons

Bombarded ions clean the surface, heat the component and diffuse the nitrogen

Glow envelops the component and nitrding starts Component cooled in atmosphere of nitrogen Anode is kept cooled by surrounding water around it Ion (Plasma) Nitriding process at Ionitech Ltd.mp4

Plasma nitriding process [Source: T. V. Rajan, C. P.

Sharma and Ashok Sharma, 2013]

Plasma nitriding Case depth depend upon current, temperature and

time of holding Advantages: Complex shapes, components of different size can

be nitrided Excellent dimensional stability Steels sensitive to tempering can be nitrided at

low temperatures Very slow white layer formation Accurate control Improved fatigue properties Cold worked steels can be plasma nitrided to get

high wear resistance Disadvantages: Equipment is complex, skilled labor required for

proper control High equipment cost Different size and shape part cannot be plasma

nitrided together Deep surfaces cannot be nitrided Besides these limitations, process is very

attractive

View of components during plasma nitriding process

Boronizing Applied to carbon and tool steels Medium: Pack or gas Pack process: Components packed in heat resistant boxes with mixture of granules or paste of

boron carbide or other boron compounds with addition of activators and dilutents

Heated in the range of 900-1000ᵒ C Boron diffuses and layers of FeB @ outer suface and Fe2B @ interior are

formed FeB phase is hard and brittle; hence not desirable. High temperature, long

treatment time and high alloys favor formation of FeB Case hardness: 1500-2100 VHN Case depth: 0.012-0.15 mm Boronizing time of 6hours for case depth of 0.15mm @900ᵒ C To optimize the performance hardening and tempering can be carried out after

boronising

Boronizing Advantages: Increases resistance of low alloy steels to sulphuric, phosphoric, and hydrochloric acid Increases resistance of austenitic stainless steel to hydrochloric acid Selective hardening is possible Can be polished to high finish Can be applied to irregular shapes Increases tool and mold life by improving resistance to abrasive, sliding and adhesive

wear Low coefficient of friction Disadvantages: Distortions due to high temperature Poor fatigue and corossion resistance Applications: Due to high hot hardness and wear resistance: Hot forging dies, wire drawing dies,

extrusion dies, straightening rolls, ingot molds etc Nozzles, plungers, gears, shafts and rollers Oil and gas components like valve components, valve fittings, metal seals, coal/oil

burner nozzles Turbine components, pump impellors, ball valves and seats, shaft protection sleeve, and

guide bars

Chromizing Applied to carbon and tool steels Medium: Pack or gas Pack process: Components packed with fine chromium powder and additives Composition of chromizing mixture: 60% Cr, < 0.1%C, 0.2% ammonium

iodide, 39% kaolin powder Heated in the range of 900-1020ᵒ C Chromium carbide formed due to diffusion of chromium Case hardness: 1500 VHN Chromising time of 12hours for case depth of 0.02-0.04mm @ 900-1020ᵒ C Types: Hard chromising: For steels with minimum % C = 0.35, hard, corossion and

wear resistant chromium layer will be formed Soft chromising: For steels with % C < 0.35, chromium carbide layer cannot

be formed. Chromium diffusion layer with 200micrometer and 35%Cr. Excellent corossion and oxidation resistance while maintaining ductility

Toyota diffusion process Developed by Toyota Central Research and Development Laboratories to

develop hard and wear resistant surface for large automotive press tools Used for die steels, tool steels, high strength steels Process: Component kept in a medium containing salt bath of proprietary composition based

on borax (sodium tetraborate) Carbide forming elements like vanadium and niobium are added in the form of

ferro-alloys. Heated at about 1050ᵒ C Carbide forming elements are diffused into the steel Quenched and tempered Case hardness: 3000VHN Carbide layer of 5-12micrometers @ 1000ᵒ C Advantages: Extremely high hardness, impact resistance and wear resistance High seizure resistance and low lubricant requirement High peel strength Applications: Press tools, shafts, screws, bushes, blades, taps, pins and plugs

Surface Hardening with no change in chemical composition

Flame hardening Applicable to steels with %C = 0.3 to 0.6 Process: Heating above upper critical temperature (here A3) by oxyacetylene flame Cooling by spraying of jet of water or immersion in water Reheating in furnace or oil bath @ 180 to 200ᵒ C for stress releiving Hardness in flame hardened steel is due to lower bainite or martensite structure Flame Hardening of Crane Wheel.mp4 Case depth: 3mm Overheating to be avoided High heating rate to avoid oxidation and decarburisation Less distortions Selective areas can be hardened Different methods of flame hardening: Spot or stationary: Shaft ends, large gears etc Progressive: Guideways, flat surfaces etc Spinning: Shafts, wheels, pulleys etc Combination of progressive and spinning: Piston rods, Rolls etc Applications: Crankshaft, axle, large gear, cam, bending roller etc

Flame hardening

Progressive flame hardening [Source: T. V. Rajan, C. P. Sharma

and Ashok Sharma, 2013]

Progressive spin hardening [Source: T. V. Rajan, C. P. Sharma

and Ashok Sharma, 2013]

Flame hardening Depth of hardening depends upon: Distance between gas flames and the component surface Gas pressures and ratio Rate of travel of flame head or component Type volume and application of quench

Induction hardening Applicable to steels with %C = 0.4 to 0.5 and some alloy steels Process: Heating done by electromagnetic induction Electromagnetic Induction.mp4 Within a short period of 2 to 5 minutes, the temperature of surface layer comes to above

upper critical temperature Quenched by jet of cold water Low temperature tempering @ 160 to 200ᵒ C Induction Hardening.mp4 INDUCTION HARDENING.mp4.mp4 Induction hardening king pin.mp4 Sometimes self tempering may also occur Skin effect: Depth of hardened layer is inversely proportional to square root of frequency

of induced current In addition to the direct heating of the skin by induced current, there is also some heating

of the core due to conduction of heat. Hence overall depth of hardness is increased Case depth: 0.5 to 6mm Applications: Crankshaft, camshaft, gears, crankpins, axles, boring bars, brake drums,

etc

Induction hardening Advantages: Orignal toughness and ductility remain

unaffected Fast heating and no holding leads to

increase in production rates No scaling and decarburisation Less distortion because of heating of only

required surface Easy control over the depth of hardening

by control of frequency of supply voltage and/or time of holding

Cleanliness of working conditions Only a light final grinding or lapping

operation may be required after hardening Disadvantages: Irregular shaped parts are not suitable for

induction hardening Not economical for small scale

production

Induction hardening process [Source: T. V. Rajan, C. P. Sharma

and Ashok Sharma, 2013]

Laser beam hardening Lens used to reduce the intensity Laser beam of 1kW can produce circular spot of diameter 0.25 to 0.50mm Process: Heating the zone to be hardened in the austenitizing range Holding to ensure adequate diffusion of carbon Self quenching Microstructure: Laser heat treated steel consists of bainite + ferrite at the surface of heated spot

and pearlite + ferrite in the interior Relationship between depth of hardening and power,

WhereY = case depth (mm)P = Laser power (W)Db = Incident beam diameter (mm)V = traverse speed (mm/s) Laser Hardening of Tool Steel.mp4 MATEX laser hardening technology.mp4 Hardening of steel axles using lasers.mp4 Case depth: 0.75mm

Laser beam hardening Independent process variables: Incident laser beam power Diameter of incident laser beam Absorptivity of laser beam by surface Transverse speed across the surface Dependent process variables: Depth of hardness Geometry of heat affected zone Microstructure and metallurgical properties of laser heat treated material Efficiency depends on absorption of light energy by work-piece Colloidal graphite, manganese phosphate, zinc phosphate and black paint are

some of the commonly used absorbent coating to avoid melting and key formation

Laser beam hardening Advantages: High production rates Effect of heat on surrounding surface is less Less time than induction and flame hardening Localized treatment is possible No external quenching is needed; necessary only for small parts No contamination Process can be controlled by computer Difficult to harden areas can be hardened No final machining needed subsequent to hardening

Electron beam hardening Used to harden the components which cannot be hardened by induction

hardening Application: Automotive transmission clutch Work piece kept in vacuum at 0.06 bar pressure Electron beam focused on work piece to heat the surface In the beginning, energy input is kept high With time, power input is reduced as the component gets heated up, to avoid

melting Computer is used to control voltage, current, beam, dwell time and focus Case depth: 0.75mm

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